We have found that estradiol is converted to 2-hydroxyestradiol by microsomes isolated from rat heart. This activity is inhibited up to 47% by an atmosphere of CO:O2 (9:1) and 32% by an antibody against NADPH cytochrome c reductase, suggesting that heart microsomes indeed possess a cytochrome P-450-dependent mixed-function oxidase. Moreover, the oxidative activity is 2-3 times greater in microsomes isolated from the atria than those from the ventricles. After i.p. administration the binding of estradiol to rat atrial protein is 3 times greater than binding to sartorius muscles, whereas binding to ventricular protein is not markedly different from that to sartorius muscle. These results support the hypothesis that the metabolism of estrogen in target tissue may help to modulate estrogenic activity in the tissue.